It is known to notify the bridged input connectors to a controller of a multistring-inverter via hardware or software to enable the controller to optimally operate the multistring-inverter whose input connectors may be bridged. Such hardware, for example, includes dip switches. Such software, for example, includes inputting the position of the switches into a control program. This input, however, may comprise mistakes. Particularly, it may be forgotten to notify an amendment to the bridging situation of the input connectors to the controller. Further, bridges between input connectors may also occur or fall away due to failures or sabotage. If, however, a multistring-inverter is operated based on wrong information about the bridges between its input connectors, high losses result as compared to the maximum feeding power possible at the same time. With long-term operation based on wrong information, restrictions to the lifetime at least of the DC/DC converters of the inverter are to be expected.
WO 2006/133714 A1 discloses a method of determining the type of connection of at least two electrical devices. Here, the input voltage at one of the input connectors of a multistring-inverter is varied by suitably controlling the associated DC/DC converter, and it is checked whether the input voltage at the other input connectors simultaneously varies between the same values. Particularly, the string connected to the respective input connector is loaded for this purpose by means of the DC/DC converter associated with the input connector. This determination will be repeated with using different input connectors and associated DC/DC converters. Further, the determination may, for example, be repeated during each startup of the inverter or every time when the inverter begins to feed electric power into an AC power grid. Thus, the determination of the type of connection of the strings according to the known method takes place prior to the actual operation of the inverter. Depending on the number of input connectors and the associated DC/DC converters, it may take some time to interrogate all different combinations of one input connector at which the input voltage is varied and of the input voltages observed at one of the other input connectors at the same time. During this determination, the voltage link is not yet fed with the maximum power available from the strings. This does not only mean a loss of principally feedable electric power but may even result in an undesired separation of the inverter from the respective AC power grid, because the respective individually loaded string or the respective individually loaded group of parallel connected strings alone is not yet sufficiently loadable for feeding electric power into the AC power grid. In this case, additional attempts of connecting the inverter to the AC power grid will be the result. These attempts stress the AC relays between the inverter and the AC power grid in an unnecessary way and thus reduce their lifetime.
There still is a need for a method of operating a multistring-inverter whose input connectors may be bridged, the method allowing for determining the string configuration without a considerable loss of electric energy feed and/or without additional attempts of connecting the inverter to an AC power grid.